1. Field of the Invention
The present invention relates to a mobile communication system and its communication method using a transmission diversity method.
2. Description of the Related Art
Generally, in a wideband code division multiple access (W-CDMA) system, a transmission diversity method where two or more antennas are provided in a base station has been used to improve the sensitivity of downstream signals from the base station to a mobile station.
On the other hand, signals from a base station are transmitted via a plurality of propagation paths, i.e., multi-paths to a mobile station. For example, signals from the base station are reflected by a building to reach the mobile station, and simultaneously, the signals are reflected by another building to reach the mobile station.
In order to improve the quality of received signals in a W-CDMA system using a transmission diversity method, in a prior art mobile station including a radio receiver circuit adapted to receive first and second signals from first and second antennas of a base station, a plurality of fingers each adapted to demodulate the first and second signals for one propagation path, and a Rake combiner adapted to combine demodulated signals of the fingers by removing phase fluctuations among the fingers are provided. Each of the fingers is constructed by a correlator adapted to descramble the first and second signals for the propagation path, a channel coefficient calculating section adapted to calculate first and second channel coefficients of the descrambled first and second signals, first and second average channel coefficient calculating sections adapted to calculate first and second average channel coefficients of the first and second channel coefficients, respectively, and a synchronous detector adapted to perform a synchronous detection upon the descrambled first and second signals using the first and second average channel coefficients (see: JP-2002-185388). This will be explained later in detail.
In the above-described prior art mobile station, in order to further improve the quality of received signals, the number of the fingers can be increased to increase the number of detected multipaths. In this case, however, the number of the correlators is increased so that the mobile station would be increased in size and also, the power consumption would be increased. On the other hand, in order to further improve the quality of received signals, the number of symbols used for calculating the average channel coefficients can be increased. In this case, however, even when the receiving path is switched from one to another by the fading phenomenon, it would be impossible to detect such switching of the receiving path so that the channel coefficients would be erroneously estimated. After all, the quality of received signals would deteriorate.
Additionally, in the above-described prior art mobile station, electric field strengths of the received signals from the antennas of the base station are detected and compared with a predetermined threshold value. When at least one of the electric field strengths is lower than the predetermined threshold value, the corresponding average channel coefficient calculating section is forcibly deactivated so that the power consumption would be decreased. Even this case, however, since the electric field strengths of the received signals from the antennas of the base station do not always represent the switching of the receiving path, it would be impossible to detect switching of he receiving path from one to another, so that the channel coefficients would be erroneously estimated.